The present invention relates to electrophotographic printing and more particularly to illumination systems in electrophotographic printing.
Many electrophotographic printing machines scan the image of an object onto the photoreceptor through an aperture located near the photoreceptor during illumination of the object by a lamp. In this type of machine, the lamp is generally cycled on and off at the rate of 120 Hz.
The light from a fluorescent lamp or other equivalent light source, running on line current, appears to the eye to be continuous. However, since it is flashing at the rate of 120 Hz, the exposure on the photoreceptor consists of many overlapping discrete images through the aperture. The nonuniform overlap of these discrete images or exposures on the photoreceptor, in many cases, results in a series of developed lines on the photoreceptor having a spatial frequency equivalent to the operating frequency of the light source and the speed of the photoreceptor. This problem is generally referred to as strobing. The problem is potentially applicable to any illumination system in which exposure is made by repeated overlapping exposures of a photosensitive surface as produced by a lamp operating at a predetermined frequency or a CRT device displaying information to a platen or an internal flashing source such as a sequential dot screen.
There are methods for compensating for this strobing effect. One method is to increase the frequency of the illumination lamp such as taught in U.S. Pat. No. 3,998,539 assigned to the same assignee as the present invention. This method, however, usually requires a high frequency power supply and adds components and complexity to the illumination system. Another method is to de-focus the aperture plate near the photosensitive surface. However, this is not always a desirable solution since too much de-focus could contribute to tracking errors. It also may be necessary to move the aperture plate in a multiple magnification system. It would be desirable, therefore, to provide a means to eliminate strobing that is reliable and simple and does not require additional components such as a high frequency power supply and readily adapted to a multi-magnification system and could compensate for non-symmetrical illumination.
The problem is often overcome by the retention of the phosphor and the required decay time in the light source. However, in such systems using Xenon or gas discharge light sources, there is frequently little or no retention time of the phosphor. Other methods of overcoming strobing, for example, in cascade development systems, are to provide an aperture width wide enough to smooth the difference illumination between points on the photoreceptor and use larger aperture lenses. This, however, can lead to tracking errors. In addition, absolute development systems, such as magnetic brush development, emphasize defects due to fringe field development. It would be desirable, therefore, to provide a simple means to overcome strobing in an absolute development system that does not decrease image resolution.
Accordingly, it is the primary object of the present invention to improve the illumination system of an electrophotographic printing machine by eliminating the effects of strobing on a photosensitive surface.
Further advantages of the present invention will become apparent as the following description proceeds, and the features characterizing the invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.